1. Field
A latent heat storage material and a process for the preparation of the latent heat storage material. More specifically, treating a mixture of expanded graphite material and a first phase change material with at least one second phase change material wherein the first and second phase change materials are immiscible to form a latent heat storage material.
2. Background
A phase change material (PCM) is capable of storing heat energy in the form of latent heat. Such materials undergo a phase transition when heat is supplied or removed, for example, a transition from the solid to the liquid phase (melting) or from the liquid to the solid phase (solidification) or a transition between a low-temperature and a high-temperature modification or a hydrated and a de-hydrated modification or between different liquid modifications. If heat is supplied to or removed from a phase change material, on reaching the phase transition point, the temperature remains constant until the material is completely transformed. The heat supplied or released during the phase transition, which causes no temperature change in the material, is known as latent heat.
The thermal conductivity of most phase change materials tends to be rather low. As a consequence, the charging and discharging of a latent heat storage device is a relatively slow process. This problem can be overcome by increasing the thermal conductivity of the phase change material by, for example, formation of a composite with a material having a high thermal conductivity. For example, according to Australian Patent Application AU 3941197 A1, a porous matrix formed of graphite can be impregnated in vacuo with a “solid-liquid” phase change material in the liquid phase. Impregnation can be performed through the use of immersion, vacuum or vacuum-pressure processes.
In addition, according to European Patent Application EP 1416027 A1, corresponding to U.S. Patent Application Publication No. US 2004/0084658, the addition of a relatively small volume (5 percent (%) or more) of expanded graphite as a heat-conducting auxiliary agent to latent heat storage materials results in a significant increase in thermal conductivity. In this aspect, the addition of a dimensional stabilizing material is not necessary. The advantages of a latent heat storage material with expanded graphite in comparison to a latent heat storage material with an equal volume content of synthetic graphite can be attributed to the nature, structure and morphology of expanded graphite.
In addition, U.S. Patent Application Publication No. US 2005/0258349 A1 describes a latent heat storage material comprising a phase change material having particles of graphite incorporated therein. At least part of the graphite is made up of flakes having a high anisotropy of thermal conductivity, a high aspect ratio and formed from one of natural graphite or anisotropic synthetic graphite.
Still further, U.S. Patent Application Publication No. US 2005/0007740 A1 describes a device for cooling heat-producing components including a heat-dissipating unit which contains at least one phase change material having a phase change temperature (TPC). The phase change material is arranged in the cooling device by its TPC according to a temperature gradient. The heat-absorbing unit may further include at least two phase change materials having different TPC which are arranged relative to one another in the cooling device by their TPC according to the temperature gradient. In this aspect, the phase change of both a phase change material with the higher TPC arranged in the vicinity of the heat-dissipating unit, e.g. a CPU, and a second phase change material with a lower TPC arranged in the more remote region of the heat sink, occurs substantially simultaneously just below a critical temperature of the CPU and therefore boosts the cooling effect.
The above described techniques lead to products having a relatively small phase transition range or a transition at a single point by arranging the phase change material according to a temperature gradient. One disadvantage of this is that the phase change materials show a very narrow temperature region in which the phase change occurs and therefore are restricted in their usage. For example, in the case of a solar heat storing material for domestic warm water preparation, a phase change material would be selected which has a transition temperature at approximately 60 degrees Celsius (° C.). In middle and north Europe, however, only a maximum water temperature of 40° C. will be reached in the winter time using standard solar panels. Thus, solar heat cannot be stored.